Manual by holding the stick for your back foot half way down.
Nose manual by holding the stick for your front foot half way up.
I did this on the Downtown Los Angeles map at the same spot I did the 40m grind achievement. You can find the spot by following the main road around, until you find a long hill in the road. I chose this spot because it's downhill for a long while meaning you'll keep enough speed to complete the full 100m easily.
Start at the top of this hill, and manual to the bottom, turn the corner to the left and take another immediate left. I stopped manualling once I reached the pedestrian crossing at the end of this road. This was enough to earn the achievement.
Here's a video I did showing the location and length of the manual I did to unlock the achievement: